This invention relates to a method for the recovery of iodine from nitric acid scrub solutions containing mercuric nitrate.
Fission-product iodine-127, iodine-129 and iodine-131 are produced in the fuel pins of a light-water nuclear reactor. Over 99% of the iodine remains trapped until the irradiated fuel is processed. During fuel dissolution, up to 99% of the entrapped iodine can be volatilized in the process off-gas. What remains in solution may be evolved later from other processes, such as evaporation and calcination of liquid wastes. Because of the long half-life of iodine-129, the recovery of these radioiodines from the waste gas streams and their ultimate storage is important to prevent any detrimental effect upon the public.
A number of processes have been developed for the recovery of these radioactive iodines from dissolver gas streams. Caustic scrubbing has been used in several reprocessing plants, but has been found to be ineffective for the removal of organic iodides. In another process the gas is passed through a very strong nitric acid solution while in still another process the off-gas is directed through a bed of silver-exchanged zeolite on which the various iodine species are chemisorbed. One of the most effective methods for the removal of radioactive iodines from gas streams is the use of nitric acid scrubbing solutions containing mercury. The nitrate salt is commonly employed because of its availability, because the gas streams often contain NO and NO.sub.2 and because nitric acid is useful in decomposing organic iodides to a recoverable form. The efficiency of the scrubber is due to the very strong complexes formed between mercury and iodine. However, this leads to the difficult problem of isolating the iodine for storage or disposal and separating the mercury for reuse. Even if not reused, disposal of mercury is to be avoided for environmental reasons.
This process is known as the Mercurex process and utilizes an 8 to 16.0 M nitric acid scrub solution containing about 0.2 to 0.4 M mercuric nitrate. At high acid concentrations the iodine in the solution can be converted from the mercuric iodide complex to the more stable (from the standpoint of iodine volatility) mercuric iodate by heating the solution to boiling before it is recycled back to the scrub column. As the acid concentrations decrease, the heating time increases and the amount of the iodine converted to the more stable form decreases rapidly. More importantly, heating the nitric acid scrub solution to the boiling temperature required for the conversion results in the volatilization of from 5 to 10% of the radioactive iodine present in the solution, requiring that the iodine must again be recovered from the gaseous state. This results in increased process costs. Alternatively, the waste scrub solutions can be mixed with concrete for long-term storage. This, however, represents a substantial volume of material for which expensive storage must be provided.